The present invention relates to a nuclear reactor for producing energy which reactor contains seed zones and blanket zones and is cooled with pressurized water; both the zones containing fissile material of plutonium and fertile material.
U.S. Pat. No. 3,154,471 discloses a pressurized water reactor designed according to the seed zone blanket zone concept, which includes subcritical blanket zones with a low concentration of fissile to fertile material and seed zones with a high concentration of fissile to fertile material. The fissile material employed is one of the elements U.sup.235, U.sup.239, Pu239, Pu.sup.241 or a combination of isotopes that can be split with thermal neutrons. The fertile material is Th.sup.232 or II.sup.238. The fertile material is Th.sup.232 or U.sup.238.
The attainment of a breeding rate exceeding 1 for thermal breeding with the use of plutonium obtained from light water reactors has not heretofore seemed possible to those skilled in the art, since, on the one hand, the number of fission neutrons per neutron absorbed in the fissile material (eta value .eta.) for plutonium.sup.239 is not high enough and, on the other hand, the simultaneously produced plutonium.sup.240 known to be a high neutron absorber. The plutonium from light water reactors was therefore considered to be usable with good breeding ratios only in fast breeders.
Breeding in thermal reactors moreover seemed possible only in the thorium cycle. This conclusion was drawn from the eta values of the conventional fission materials at 0.025 eV, which values are: 2.3 for U.sup.233 ; 2.077 for U.sup.235 ; and 2.109 for Pu239. Since the eta value should lie above 2 to permit breeding, the attainable range with U.sup.235 and Pu.sup.239 was considered to be too small.